(a) Field of the Invention
This invention relates to an oil and gas exploration system and method of detecting gases in the atmosphere and more particularly, but not by way of limitation, to detecting hydrocarbon gases using field-scale, differential absorption lidar (DIAL) sensing techniques operating in a mid-infrared, 2 to 5 micrometers, spectral range.
(b) Discussion of Prior Art
Gases, that are trapped or generated within the earth, can escape and travel through the earth to the earth""s surface and then into the atmosphere. While in the earth, the gases come in contact with or are created by deposits of hydrocarbons and thereby acquire some signature of the deposit. The atmosphere can be monitored for gases that are associated with the deposits of hydrocarbons. The subject invention described herein addresses the measurement of gases associated with a potential oil and gas deposit. The gas concentrations are then mapped over a survey area and the maps are analyzed for concentration anomalies as described herein. The gas anomalies are interpreted along with other exploration data to evaluate the value of the underground deposit.
An association of gases detected in the atmosphere with a hydrocarbon deposit may be direct or indirect. An example of a direct association is the release of hydrocarbon gases to the atmosphere from subsurface oil and gas deposits. The association is direct in that the gas itself is emitted into the atmosphere, albeit with a potentially modified composition.
Methane is produced from the thermal or biological breakdown of coal. The gas detected (methane) is not the same as the natural resource (coal), so the term xe2x80x9cindirectxe2x80x9d is used to describe this association. The term indirect association does not imply that the scientific basis for the association is weak. The process of converting coal to methane is well characterized in the scientific literature. Also, coalbed methane is a new and expanding area of production for the natural gas industry.
In the discussion of the subject invention, the term xe2x80x9ctarget gasesxe2x80x9d is used to indicate gases that are associated either directly or indirectly with deposits of hydrocarbons. The measured atmospheric concentrations of target gases form the basis of the new exploration tool as described herein. Target gases must have some uniqueness to their association with the hydrocarbon deposit. For example, methane is produced in a number of ways. It may occur in the atmosphere as a result of emission from a hydrocarbon deposit, emission from a coal deposit, emission from wetlands with active populations of methane producing bacteria, emission from a leaking natural gas pipeline, etc. Sources of methane other than the hydrocarbon deposit are said to be environmental interferences. Environmental interferences complicate the association between a target gas and the hydrocarbon deposit and will vary in magnitude and type with standard geological factors such as soil type, hydrology, subsurface structure and composition as well as atmospheric conditions, weather and land use.
A non-unique gas such a methane is a useful target gas for fossil fuels if combined with additional measurements to create a unique association. Examples of additional measurements are: concentrations of other gases such as ethane, propane, etc., isotopic composition of the methane, type of vegetation present, soil moisture, proximity of pollution sources and wind direction. Individual gases or gas combinations that have very unique associations with the hydrocarbon deposit provide the most valuable exploration signatures.
Laser absorption spectroscopy (LAS) is a sensitive means of quantifying molecular concentrations in a variety of situations not amenable to other techniques. A main advantage of LAS is that the measurements is done xe2x80x9cin situxe2x80x9d which enables rapid measurements with good spatial resolution in harsh environments such as plasma, high vacuum and chemical reaction chambers. For an absorption experiment, the ratio of the transmitted beam intensity to the initial beam intensity, I(∀,x)/Io(∀,x=O), is related to an absorber concentration, n, by Beer""s Law.             I      ⁡              (                  v          ,          x                )                            I        o            ⁡              (                  v          ,                      x            =            0                          )              ⁢      ⅇ                  -                  σ          ⁡                      (            v            )                              ⁢      nx      
The molecular cross-section at frequency, ∀, is denoted "sgr"(∀) and the path length over which the laser travels by x. For any given signal to a noise ratio (SNR) for the measurement of I/Io, the measurement sensitivity can be increased by increasing the path length. This patent application includes a differential absorption lidar (DIAL) which samples long paths through the atmosphere.
A wide range of instruments have been developed which successfully detect most trace gases in the atmosphere. These instruments can be loosely categorized into point techniques which sample air at a specific point in space and remote sensing systems such as the numerous satellite-based systems which provide large-scale measurements of gas concentrations. There are numerous types of gas sources which, because of their unique spatial and temporal properties, cannot be accurately characterized by these techniques. For example, an underground reservoir which might contain methane, carbon dioxide or gases from polluted ground waters, can leak intermittently from many points along a surface fracture. Monitoring emissions from such sources requires a system which can measure minute concentrations quickly and over long paths. Long path differential absorption lidars (DIALs) meet these requirements.
There are a number of prior art patents that describe oil and gas exploration systems that include means for detecting trace gases in the atmosphere. Some of these systems operate in the microwave or the ultraviolet wavelength region. These systems are unlike the subject invention which operates in the mid-infrared wavelength range.
The following patents are mentioned since the systems described therein operate in the mid-infrared wavelength region for detecting hydrocarbon gases. In U.S. Pat. No. 4,450,356 to Murray et al., a frequency-mixed CO2 laser beam is used for remote detection of gases in the atmosphere. The laser beam system uses frequency doubling and frequency summing in crystals to produce wavelengths near 3 micrometers. This type of CO2 laser system""s wavelength is not continuously tunable.
In U.S. Pat. No. 4,489,239 to Grant et al., a portable remote laser sensor is described for detecting methane gas. The system requires the use of two lasers. The two lasers operate at two different wavelengths, each of which is fixed. The detector must be kept at liquid nitrogen temperatures and does not operate at room temperatures. Further, the two lasers are not tunable and are used for detecting methane only. In U.S. Pat. No. 4,871,916 to Scott, a laser system, using glass lasers, is described for detecting explosive amounts, 40,000 parts per million (ppm), of methane only. In this system, the wavelength region is near 1.3 micrometers and the lasers are not tunable.
In U.S. Pat. Nos. 5,157,257 and 5,250,810 to Geiger, a mid-infrared light hydrocarbon DIAL lidar is described. The system uses six distinct coherent beams formed by six different lasers. The beams are combined into a single transmitted beam. While the six lasers are tunable, they include crystals which are easily damaged by high energy laser pulses. The complexity of this type of DIAL is not conducive to use in the field. Also, the spectral width is too broad to resolve the absorption bands of many key gases.
None of the above mentioned patents disclose or describe the unique features, structure, function and method steps of the subject oil and gas exploration system and method of detecting trace amounts of hydrocarbon gases in the field and in the mid-infrared region using a DIAL system with a single spectroscopic grade laser source.
In view of the foregoing, it is a primary object of the subject invention to provide a unique oil and gas exploration tool which is able to detect sub parts per million (ppm) amounts of hydrocarbon gases in the atmosphere.
Another object of the invention is the subject DIAL system can be used for mapping a plurality of target gases over a selected survey area. The maps are analyzed for concentration anomalies that indicate the presence of a potential hydrocarbon reservoir.
Yet another object of the DIAL system is its ability to achieve high energy laser pulses, in the mid-infrared wavelength range, 2 to 5 micrometers, with excellent spatial and spectral quality. The term xe2x80x9cspectroscopic grade laserxe2x80x9d used herein refers to lasers that produce light with a spectral width at least a factor of ten less than the spectral width of the target gas absorption bands used to make the DIAL measurements. Good spectral control enables the system to detect trace amounts of specific gases, such as ethane, methane and propane.
Still another object and key feature is the DIAL system is tunable for choosing appropriate wavelengths to measure different gases. Because the system is tunable, absorption bands of interference gases, such as water vapor and CO2, can be avoided and the wavelengths can be chosen resulting in the most sensitive detection of a particular target gas.
A further object of the invention is the system allows trace gases to be accurately detected over one mile (two mile round trip) paths and greater and in a time span of under one minute.
Another object in the invention is the system is housed in a mobile trailer for transporting to various field locations. The system is operable and reliable for field applications and under harsh weather conditions. Also, the mid-infrared laser beam emitted is eye safe at all energy levels and at all wavelengths.
The system includes a differential absorption lidar (DIAL) system with a spectroscopic grade laser, a laser light and a light detector. The laser light is broadly tunable in the mid-infrared wavelength range, 2 to 5 micrometers, of the electromagnetic spectrum from which the appropriate wavelengths to measure different hydrocarbon gases and avoid absorption bands of interference gases can be chosen. The DIAL system with the laser light source is housed in a mobile platform for field operation. Also, the DIAL system can be used on an airborne platform for airborne survey applications. The laser light has sufficient optical energy to measure atmospheric concentrations of a selected gas over a path as long as a mile and greater. The detection of the target gas is based on optical absorption measurements at specific wavelengths in the open atmosphere. Light that is detected, using the light detector, contains an absorption signature acquired as the light travels through the atmosphere from the laser source and back to the light detector. The absorption signature of each target gas is processed and then analyzed to determine the gas concentration in the atmosphere. The target gas concentrations are mapped and analyzed to determine presence of concentration anomalies.
These and other objects of the present invention will become apparent to those familiar with the different types of hydrocarbon gas detection systems and methods when reviewing the following detailed description, showing novel construction, combination, and elements as herein described; and more particularly defined by the claims, it being understood that changes in the embodiments to the herein disclosed invention are meant to be included as coming within the scope of the claims, except insofar as they may be precluded by the prior art.